It's true that it isn't something we can say definitively. It's more than a philosophical belief, however.

There isn't any evidence that there is anything happening that's not biological despite 1000's of years of searching for that evidence.

And there is all kinds of evidence that biological processes can explain our experience. More all the time.

It's not definitive in the sense that we're anywhere near completely understanding our biology.

But it's the most likely explanation, given what we do know, but such a huge margin that there is no real alternative explanation outside of mythology.